A new account of the Fukushima Daiichi nuclear disaster provides more detail of utility crews' desperate battle to head off reactor meltdowns amid a maelstrom of darkness, aftershocks, explosions, radiation exposure and mortal danger. But the report by the U.S. industry performance monitor still does not resolve why some critical valves and controls failed at the stricken plant.

The matter-of-fact language of the Institute of Nuclear Power Operations (INPO) report describes workers' frantic, makeshift efforts at operating crucial valves and instrumentation with truck batteries; hauling massive emergency cables across flooded passageways where manhole covers were dislodged; wading through irradiated water; being driven back from reactor chambers by bursts of steam and sudden spikes in radiation; and subsisting each day on just biscuits and a bowl or two of noodles because of food shortages.

"Because of the tsunami and earthquake damage to the surrounding communities, little outside assistance was initially available. Some workers lost their homes and families to the earthquake and tsunami, yet continued to work," the authors wrote.

Two workers were trapped inside a plant turbine building and killed by floodwaters. Sixteen workers and Japan Self-Defense Force members were injured by explosions. Two were treated for high exposure, and others were exposed to radiation beyond emergency limits.

"The report reinforced that it was hell on earth for those operators. You could hardly write a scenario that was more horrific than what they were facing," said Lake Barrett, former head of the Energy Department's Office of Civilian Nuclear Waste Management and site manager for the stabilization and cleanup at the Three Mile Island reactor after the 1979 accident there.

The INPO chronology of the accident's first four days does not offer recommendations for new safety regulations for U.S. reactors. "Most of the information in the report has been previously released, but in a piecemeal fashion," the industry's Nuclear Energy Institute said in releasing the report.

NEI Senior Vice President and Chief Nuclear Officer Tony Pietrangelo said, "It is important that we all work from the same set of facts in determining the appropriate response. It is of paramount importance that we learn from it and take our facilities to even higher levels of safety and preparedness."

The report is likely to add to the debate about how U.S. reactor regulation and practices should be changed in light of the Fukushima disaster. The Nuclear Regulatory Commission, after concluding that U.S. plants remain safe, has begun initial regulatory actions to strengthen safety defenses and reevaluate worst-case earthquake threats to U.S. reactors. Any remedial action based on the earthquake review could take years to be ordered and completed, industry officials say.

Planners missed the worst case

The plant operator, Tokyo Electric Power Co. (TEPCO), and Japanese regulators failed to anticipate the impact of a worst-ever tsunami on the plant's defensive systems, INPO's report notes.

"The tsunami design basis for Fukushima Daiichi [based on the maximum anticipated threat] considered only the inundation and static water pressures, and not the impact force of the wave or the impact of debris associated with the wave," INPO reported. Flooding and debris disabled water intakes that were essential to cool the reactor and emergency generators.

In 2002, TEPCO voluntarily reassessed the plant's defenses against tsunamis, concluding that the risk of higher tsunamis had to be considered. The defenses included a breakwater barrier that ranged in height from 18 to 33 feet.

"The breakwater was not modified when the new tsunami height was implemented because it was not intended to provide tsunami protection, but rather to minimize wave action in the harbor," INPO said. At least one of the tsunami waves that struck on March 11 was above 46 feet in height, five times greater than officials anticipated. It swamped the facility, disabling backup diesel generators that were essential to maintain cooling of the reactors, which shut down automatically following the quake.

A still-unresolved issue for U.S. nuclear operators and the NRC is how to strengthen plants in this country against the prolonged power loss that Fukushima suffered.

Without alternating-current power and backup battery reserves, the Fukushima crews were in a race against time as they labored to maintain water cover over the reactor fuel assemblies with jury-rigged attempts. The report outlines how that race was lost, leading to the buildup of explosive hydrogen gas. Steam and gas pressure soared to double normal levels, effectively blocking lower-pressure water from fire trucks and emergency pumps from reaching the core. Some pressure relief valves that could have opened automatically failed to do so, a failure that is still under study.

The core in Unit 1 may have become uncovered three hours after the earthquake, and fuel damage might have commenced approximately 1.5 hours later. The first hydrogen explosion, at Unit 1, occurred a day after the earthquake. Workers had been just six minutes away from reconnecting power to Unit 2 when the massive blast blew the top off Unit 1, damaging Unit 2 and littering the area with radioactive debris. "The explosion significantly altered the response to the event and contributed to complications in stabilizing the units," INPO said.

"One of the things that is really important in lessons learned -- we knew it before, but this really reinforces it -- high containment pressures are a real problem in this design to get low-pressure water in," Barrett said. "They realized later in the [first] day that they weren't getting core cooling because the radiation levels were so high." But with all power supply lost, the improvised water supply remedies did not work in time to prevent the fuel rods from becoming uncovered and melting down.

Failure of basic emergency instruments

A key focus of the NRC's Near Term Task Force report on the Fukushima accident was the need to strengthen emergency equipment that could overcome the loss of outside and internal power and increase training for coping with extreme emergencies.

The INPO report cites critical shortages of emergency equipment at Fukushima.

Portable generators were located. "However, damaged roads and congested traffic prevented the generators from reaching the site quickly," INPO says.

"Helicopters were considered, but the generators were too large and heavy to carry." Some mobile generators began arriving late on March 11, but could not immediately be connected because of the damage and debris. From the generators to connect to the plant, workers had to lay electric cables, each of them 656 feet long, weighing 1 ton, across flooded spaces. "The force of the tsunami had blown manhole covers off, resulting in unmarked openings in the ground" that could have swallowed up a worker, INPO said.

The failure of crucial instrumentation because of the power outage -- another issue before the NRC -- left operators unable to verify critical water, temperature, pressure and radiation levels. "The lack of available containment pressure indications early in the event may have prevented the operators from recognizing the increasing pressure trend and taking action earlier in the event," the report says.

But even more basically, workers didn't have enough portable radios to communicate with command centers or enough dosimeters to measure individual workers' radiation exposures.

Operators were sent into Unit 2 to check the condition of an emergency cooling system, but were slowed by the breathing equipment they had to wear. Water in the building came nearly to the top of their protective boots, and they had to retreat. Lacking radios, they had to return to the control room to report the situation.

Delaying a decision to vent Unit 1

The INPO report describes an extended and complex decisionmaking process that unfolded before orders were given to vent the reactor containment structures to relieve dangerous pressures. The adequacy of emergency planning at U.S. reactor sites is another issue awaiting NRC action.

A decision to vent Unit 1 was made at 1:30 a.m. on the second day, March 12. But since venting would release radioactivity into the environment, the action was delayed to assure that public evacuation plans had been completed, INPO said.

At 6:50 a.m. on March 12, the Ministry of Economy, Trade and Industry ordered TEPCO to vent Unit 1. Prime Minister Naoto Kan arrived at the plant shortly after 7 a.m. in the midst of the venting decisionmaking. The established procedure said that a venting decision "should be coordinated with local governments and that the station should be knowledgeable about the status of evacuations. These statements had been interpreted as providing guidance to verify evacuations were completed before venting," INPO said.

The decisions framework wasn't clear, Barrett said, and the Japanese culture of decisionmaking by consensus slows action further. "I think it exacerbated the situation."

But once the venting go-ahead was given, crews were unable to begin venting until 2:30 p.m., almost 24 hours after the accident began, and by that time, it was too late to prevent hydrogen from escaping the containment through some still-unconfirmed breach, and gather at the top of Unit 1. The explosion occurred one hour later.